The potential estrogen receptors in the liver, hypothalamus, pituitary and uterus are being investigated. We have previously found that the adult female rat has estrogen specific binding proteins in the supernatant fractions. These binding proteins are being characterized and their potential biologic functions evaluated. The hypothalamic and pituitary receptors are most likely responsible for inhibition of ovulation by the estrogenic component in oral contraceptives. The estrogen binding liver protein might be involved in mediating serious side effects of the oral contraceptives by modulating the synthesis of selective plasma proteins.